area51

I've been using StackOverflow for years as a programmer, and never before scrolled to the bottom to see the whole community of sites.  Not only is there stackoverflow for plumbers and scifi/fantasy buffs, but http://area51.stackexchange.com/ lets you join or start a community of experts for a new stackoverflow site.

Its interesting that its not just platform software anyone can use, but that the community/reputation/expert factor is an intrinsic part of the thing.  Its not just the software, apparently.